Spatial separation of small and large grains in the transitional disk around the young star IRS 48

TL;DR

This study uses spatially resolved mid-infrared imaging to analyze the distribution of small and large dust grains in the transitional disk around IRS 48, revealing a ring structure and PAH emission filling the gap.

Contribution

It provides detailed spatial analysis of dust grain distribution and PAH emission in IRS 48's disk, suggesting mechanisms for gap formation and disk evolution.

Findings

The 18.7 micron ring peaks at 110 AU with a 60 AU gap.

PAH emission is centrally concentrated and much stronger than typical for similar stars.

The inner disk is cleared of micron-sized dust but retains PAHs.

Abstract

We present spatially resolved mid-infrared images of the disk surrounding the young star IRS 48 in the Ophiuchus cloud complex. The disk exhibits a ring-like structure at 18.7 micron, and is dominated by very strong emission from polycyclic aromatic hydrocarbons at shorter wavelengths. This allows a detailed study of the relative distributions of small and large dust grains. Images of IRS 48 in 5 mid-infrared bands from 8.6 to 18.7 micron as well as a low resolution N-band spectrum are obtained with VLT-VISIR. Optical spectroscopy is used to determine the spectral type of the central star and to measure the strength of the Halpha line. The 18.7 micron ring peaks at a diameter of 110 AU, with a gap of ~ 60 AU. The shape of the ring is consistent with an inclination of i = 48 +- 8 degrees. In contrast, the 7.5-13 micron PAH emission bands are centered on the source and appear to fill the gap within the ring. The measured PAH line strengths are 10-100x stronger than those typically measured for young M0 stars and can only be explained with a high PAH abundance and/or strong excess optical/UV emission. The morphology of the images, combined with the absence of a silicate emission feature, imply that the inner disk has been cleared of micron-sized dust but with a significant population of PAHs remaining. We argue that the gap can be due to grain growth and settling or to clearing by an unseen planetary or low-mass companion. IRS 48 may represent a short-lived transitional phase from a classical to a weak-line T Tauri star.